Also, as an addition, how much of what we "know" about Norse mythology was actually made up by Christians post-1000? I've heard quite frequently that most of what we know was written down by people like Saxo, way later than the conversion of Scandinavia.

The worship of Greek gods could be said to have declined somewhere between Emperor Constantine making Christianity the official Roman religion, and the time when Emperor Theodosius banned all religions except Christianity in the Roman Empire - Greece being part of the Roman Empire at that time, and continuing to be the Eastern Roman Empire for a long time afterwards.

As for reasons - the outright banning of it would be a pretty good one. Also, their temples would have been destroyed, or remade into Churches (this was a common practice).

See Proscription of Hellenistic religion for more information.

Edit: Other people have corrected me below, I was wrong about how these things unfolded exactly, I am not a professional historian, I just figured the answer is so obvious I don't need to be. But it seems it was not.